A communication system can be seen as a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system. The communication may comprise, for example, communication of voice, data, multimedia and so on. The communication system may be circuit switched or packet switched. The communication system may be configured to provide wireless communication.
At least some signaling is typically required in order to control a communication session. Communication between the user equipment and the entities of the communication network and the signaling can be based on an appropriate communication protocol or protocols.
Feedback information is needed for controlling the transmission in many communication systems. Especially in frequency division duplex (FDD) systems, where transmission and reception occur at different frequencies, it may not be possible to deduce channel information of a channel, on which a device transmits to a second device, based on a channel, on which signals are received from the second device. In time division duplex (TDD) systems, transmission and reception occur at the same frequency, so channel information is usually readily available.
Feedback information may relate to transmission power control or to control of transmission more generally. Often feedback information is related to a continuous quantity, or otherwise an ordering is induced. This means that one may define a metric in the space of feedback symbols relaying feedback information so that some possible feedback symbols are closer neighbors than some others. If the feedback is based on a continuous quantity, the corresponding metric may be taken as a Euclidean metric in the space of the continuous quantities. For example, in many transmit diversity methods, where the phase difference between two or more signals sent from two or more antennas is adjusted, feedback information is needed for proper adjustment of the phase difference. Feedback information may also be used to control both phase and amplitude of transmission. In these cases, the metric defining proximity may be taken as the Euclidean metric on the plane.
When feedback information is conveyed from the receiving end of the communication link to the transmitting end, digital communication is often used. For this, the feedback information describing continuous quantities has to be quantized using a predefined quantization method. Such quantized numbers are often called feedback symbols. Typically feedback symbols are then described in terms of a set of binary feedback words. Information about this feedback symbol is sent to the transmitter by sending the determined feedback word as feedback signaling. After receiving the binary feedback signaling, the transmitter controls transmission. Often the transmitter constructs the feedback symbols from the received feedback signaling i.e. quantized estimates of the continuous quantities (phase, amplitude, complex number) and uses them directly to control transmission. For example, phase shift keying modulation symbols may be used to indicate how to modify the phase difference of two signals sent to a receiver. Usually a mapping called Gray encoding is used for transmitting signaling information and for transmitting feedback information.
Usually information to be transmitted over a wireless link is protected against transmission errors. This protection can be performed, for example, by adding some error detection/correction codes or by adding redundancy by partially repeating the information to be transmitted. Feedback signaling is typically unprotected for enabling fast transmission of the feedback information and for saving transmission resources. This means that feedback signaling is error-prone. A bit error in feedback information may result in the use of a wrong feedback symbol in transmission control. For example in WCDMA (Wideband Code Division Multiple Access) standardization it is assumed that that the bit error rates (BER) for unprotected feedback signaling is about 4-10%.
To overcome at least some of the problems relating to bit errors in transmitting feedback information, a concept of verification has been introduced in some diversity methods, for example in Mode 2 diversity in WCDMA system. In verification the receiver tries, after sending feedback information, to deduce from the signals sent by the transmitter which feedback symbol the transmitter is employing in the transmission control. The verification is typically based on pilot signals (or other signals whose information content is known to the receiver) and on the signals whose transmission is controlled using the feedback information. As the verification is based on estimation of various signals, possible measurement errors and poor estimation accuracy affect the accuracy of verification. In WCDMA mode 2, verification typically detects only half of the situations, where a bit error in a feedback word has caused a wrong feedback symbol to be used in transmission control. It should be noted that in those cases, where verification does not detect the use of a wrong feedback symbol, the receiver typically cannot receive information properly. In such cases the use of feedback actually degrades the performance of the system.
Furthermore, errors in feedback information may cause a transmitter to use an unnecessary high transmission power for a certain receiver. This typically causes interference to other receivers.
Regarding WCDMA, it is furthermore noted that WCDMA has two different modes: mode 1 and mode 2. Signal to a receiver may be sent from two antennas of a base station of WCDMA. Either the phase difference of the two signals relating to the two antennas is adjusted while keeping the ratio of the transmission powers fixed (mode 1), or both the phase difference and the amplitudes are adjusted (mode 2). In mode 1, 2-bit feedback words indicate one of four possible phase differences. In mode 2, 4-bit feedback words are needed. Three bits of a feedback word indicate one of eight possible phase differences, and one bit indicates one of two possible amplitude ratios. As mode 2 provides a more sophisticated feedback method, it is expected that the system performance is enhanced by using mode 2 instead of mode 1.
There are thus problems relating to the transmission of feedback information over a noisy channel.
An aim of the present invention is to provide a more reliable method for transmitting feedback information.